The present invention relates to a transparent substrate, in particular made of glass, provided with a thin-film stack including at least one metallic layer with infrared reflection properties, in particular a low-emissivity layer, arranged between two dielectric-based coatings.
The main application which the invention addresses is the use of such a substrate for the manufacture of thermal-insulation and/or solar-protection windows.
These are intended to be fitted both to buildings and to vehicles, in particular with a view to decreasing the air-conditioning work load and/or reducing excessive overheating entailed by the ever-increasing size of glazed areas in rooms and passenger compartments.
One type of thin-film stack well known for giving a transparent substrate thermal properties, in particular low-emissivity properties, which is suited to the aforementioned required application consists of a metallic layer, in particular made of silver, arranged between two dielectric-based coatings of the metal oxide or nitride type. This stack is, in the usual way, manufactured using a sequence of depositions carried out using a vacuum technique, for example cathode sputtering, where appropriate assisted by a magnetic field.
A metallic overcoat having a protective role for preventing degradation of the silver may also be provided in this stack.
In this type of stack, the silver layer essentially dictates the thermal, solar-protection and/or low-emissivity performance of the final window, whereas the dielectric layers primarily affect the optical appearance of the window obtained through interference. They furthermore have a function of protecting the silver layer against chemical and/or mechanical attack.
The improvements made to windows provided with stacks of the aforementioned type have so far allowed their field of application to be increased, while allowing them to keep a satisfactory level of thermal and optical performance. Regarding the latter point, however, colorimetric appearance is susceptible of yet further improvement, in particular with a more neutral colour in reflection.
The object of the invention is therefore to provide a substrate provided with a thin-film stack of the aforementioned type, having an improved colorimetric appearance, in particular with a more neutral colour in reflection.
To that end, the invention relates to a transparent substrate, in particular made of glass, provided with a thin-film stack including at least one metallic layer with infrared reflection properties, in particular a low-emission layer, arranged between two dielectric-based coatings, above the metallic layer. According to the invention, the dielectric-based coating has the sequence of layers deposited in this order:
a) layer(s) with material(s) with refractive index ni-2 of at most 2.2
b) layer(s) with material(s) with refractive index ni-1 at least 0.3 less than that of the last layer(s) ni;
c) last layer(s) with material(s) with refractive index ni substantially equal to ni-2.
It is to be pointed out that, in the scope of the invention, the term xe2x80x9clast layer(s) in the stackxe2x80x9d is to be understood as meaning the layer(s) in the stack furthest away from the substrate, which comprises the one directly in contact with the environment such as air.
The solution according to the invention gives an advantageous yet very unexpected result.
By way of explanation, until now in order to have a notable antireflection effect for the type of stack which the invention addresses, it had been necessary to obtain a dielectric-base upper coating in which the layer directly in contact with the protective metallic layer was a layer based on a material with high refractive index, which material had the advantage of xe2x80x9cantireflectingxe2x80x9d the metallic layer with infrared reflection properties. This upper coating was preferably terminated by an xe2x80x9cindex gradingxe2x80x9d with a last layer in contact with the environment such as air, with a refractive index closest to that of the latter, of the order of 1.7.
This was justified by the fact that the reflection at the interface between the last layer in the stack and the air is all the less as the difference in index between the material and the air decreases.
Surprisingly, it has been found that, for the type of stack addressed by the invention, the desired antireflection effect was obtained in spite of the presence of a last layer with high refractive index in contact with the environment such as air, by using the sequence according to the invention.
Furthermore, consequently, the substrates provided with the stack according to the invention have a neutral colour in reflection, even in configurations where the metallic functional layer, such as silver, has a large geometrical thickness.
The windows obtained by incorporating such substrates therefore both are highly aesthetic and have very good performance from a thermal point of view.
There are many advantages resulting from the sequence of the stack according to the invention compared with the sequences according to the prior art, according to which it is necessary to have index grading in order to obtain a satisfactory colour in reflection.
Specifically, because, according to the invention, the last layer has a high refractive index, it is readily possible to choose a constituent material for it which considerably improves the mechanical durability of the stack, such as a material based on SnZnOx, or which gives it a xe2x80x9ctemperablexe2x80x9d character without modification to its properties, such as a material based on Si3N4.
The functional metallic layer is advantageously made of silver. Its thickness may be selected between 7 and 20 nanometers, in particular between 9 and 15 nanometers, when it is desired to obtain windows with low emissivity and high optical transmission (in particular a TL of at least 70 to 80%), particularly for those intended to be fitted to buildings in cold countries. When the desire is for reflective windows with a solar-protection function, which are intended rather to be fitted to buildings in hot countries, the silver layer may be thicker, for example between 20 and 25 nm (which clearly has the consequence of having windows with much lower optical transmission, for example less than 60%).
Preferably, provision may be made for the stack according to the invention to have a protective metallic layer placed immediately above and in contact with the layer with infrared reflection properties.
The protective layer provided is advantageously based on a single metal selected from niobium Nb, titanium Ti, chromium Cr or nickel Ni or an alloy of at least two of these metals, in particular an alloy of nickel and chromium (Ni/Cr), and has a geometrical thickness of less than or equal to 2 nanometers. According to this variant, the metal or the alloy constituting the layer may be doped with palladium Pd. It fulfils its role as a xe2x80x9csacrificialxe2x80x9d layer with the aim of protecting the functional layer in the case of depositing the next layer by reactive sputtering.
The dielectric-based coating lying underneath the metallic layer with infrared reflection properties is advantageously a superposition of at least two layers. It may be two layers, notably the superposition of two metallic oxide layers or the superposition of a nitride layer like AlN or Si3N4 and a metallic oxide layer like SnO2, ZnO, TiO2. Preferably, there is a direct contact between siad coating and said metallic reflecting layer. A preferred embodiment consists of a coating having a wetting layer based on zinc oxide ZnO, optionally doped with aluminium ZnO:Al, in contact with the said layer with infrared reflection properties. The coating contains preferably two layers including this wetting layer. The geometrical thickness of the wetting layer is preferably between 5 and 40 nanometers, in particular between 15 and 30 nanometers. With such thicknesses, further to its wetting function, it can contribute to adjusting the optical appearance of the stack in combination with the first dielectric coating lying above the functional layer. The wetting layer can advantageously be based on partly crystallized zinc oxide.
Such a layer makes it possible to avoid penalizing the stack from an optical point of view in the case when the carrier substrate subjects it to a heat treatment of the bending or tempering type.
According to one highly advantageous characteristic of the invention, the last layer(s) in the stack may furthermore be a metal oxide layer selected from one of the following materials: tin oxide SnO2, zinc oxide, mixed oxide of tin and zinc SnZnOx. It may also be a metal nitride layer, optionally an oxygen-diffusion barrier, selected from one of the following materials: silicon nitride Si3N4, optionally doped with aluminium Si3N4:Al, aluminium nitride AlN.
Lastly, it may a carbide layer, optionally an oxygen-diffusion barrier, selected from one of the following materials: SiC, TiC, CrC, TaC.
As mentioned above, such a layer makes it possible to improve the durability of the stack and/or give it a xe2x80x9ctemperablexe2x80x9d character.
This last layer preferably has a geometrical thickness of between 5 and 20 nanometers. Advantageously, the layer placed immediately underneath and in contact with one of the last layers in the stack has a refractive index ni-1 of less than 1.75.
This layer is preferably based on silicon oxide SiO2 or aluminium oxide Al2O3 or a mixture of these two oxides Al2O3:SiO2.
According to an additional characteristic of the invention, the layer underlying the layer placed immediately underneath and in contact with one of the last layers has a refractive index ni-2 close to that of the last layer in the stack, preferably of the order of 2.
By way of preferred illustration, the stack of layers meeting the criteria of the invention is of the type:
Glass/SnO2 or Si3N4:Al or AlN/ZnO or ZnO:Al/Ag/Ti or NiCr/ZnO or SnO2/SiO2 or Al2O3 or SiO2:Al2O3/SnO2 or ZnO or SnZnOx or AlN or Si3N4:Al or (AlN/Si3N4:Al) or (Si3N4:Al/AlN) or (SnO2/SnZnOx)
The substrate according to the invention is also noteworthy in that it has an emissivity xcex5 of at most 0.05.
The invention which has just been described is susceptible of very many applications. This may involve, in particular, low-emission or solar-protection multiple glazing, in particular double glazing, having the substrate defined above, the stack of layers being at faces 2 and/or 3, and where appropriate, at face 5.
It also relates to low-emission or solar-protection double glazing having at least one substrate defined above, noteworthy in that it has an optical transmission TL of at least 72%.
Such double glazing having two panes of glass is also noteworthy in that it has a coefficient K less than or equal to 1.4 W/K.m2 when the two panes of glass are separated by a layer of air, or less than or equal to 1.1 W/K.m2 when the panes of glass are separated by a layer of argon.